One of the most important things to remember when accompanying another musician is that you are not playing alone. In fact, you are not the focus of the performance. With that in mind, prepare to accompany by using a low volume technique. For example, play every dynamic (musical instruction determining playing volume) one step lower. If the music says to play mezzo forte (medium loud), play medium soft. While practicing the piece, keep this concept in mind. Don't put your fellow musician in a position where he must play louder because you are covering his sound with your own.
Also play along with recordings of the piece you are preparing. Set the recording at a certain volume and never play at such a volume that you cover the sound of the recording. This can be difficult to master at first, but will do wonders when rehearsing with your musician. Stay at a volume not exceeding the musician, but always try to match their dynamics. If the musician you are accompanying plays louder, so should you.
While accompanying a musician, watch them as much as possible. Of course, this may require a great deal of memorization on your part. However, memorizing music comes naturally to most pianists and it should not inhibit your preparation. To develop awareness, tape a piece of paper on the wall in your practice area. Watch that piece of paper as you practice. If there is a video recording of the piece you are preparing, playing this on a television while practicing can be a fantastic way to prepare. Do not sacrifice accuracy for focus, however. This method is suitable only when you are familiar with the piece.